'\" et
.TH LOCKF "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
lockf
\(em record locking on files
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>
.P
int lockf(int \fIfildes\fP, int \fIfunction\fP, off_t \fIsize\fP);
.fi
.SH DESCRIPTION
The
\fIlockf\fR()
function shall lock sections of a file with advisory-mode locks. Calls
to
\fIlockf\fR()
from threads in other processes which attempt to lock the locked file
section shall either return an error value or block until the section
becomes unlocked. All the locks for a process are removed when the
process terminates. Record locking with
\fIlockf\fR()
shall be supported for regular files and may be supported for other
files.
.P
The
.IR fildes
argument is an open file descriptor. To establish a lock with this
function, the file descriptor shall be opened with write-only
permission (O_WRONLY) or with read/write permission (O_RDWR).
.P
The
.IR function
argument is a control value which specifies the action to be taken. The
permissible values for
.IR function
are defined in
.IR <unistd.h> 
as follows:
.TS
box tab(!) center;
cB | cB
l | l.
Function!Description
_
F_ULOCK!Unlock locked sections.
F_LOCK!Lock a section for exclusive use.
F_TLOCK!Test and lock a section for exclusive use.
F_TEST!Test a section for locks by other processes.
.TE
.P
F_TEST shall detect if a lock by another process is present on the
specified section.
.P
F_LOCK and F_TLOCK shall both lock a section of a file if the section
is available.
.P
F_ULOCK shall remove locks from a section of the file.
.P
The
.IR size
argument is the number of contiguous bytes to be locked or unlocked.
The section to be locked or unlocked starts at the current offset in
the file and extends forward for a positive size or backward for a
negative size (the preceding bytes up to but not including the current
offset). If
.IR size
is 0, the section from the current offset through the largest possible
file offset shall be locked (that is, from the current offset through
the present or any future end-of-file). An area need not be allocated
to the file to be locked because locks may exist past the end-of-file.
.P
The sections locked with F_LOCK or F_TLOCK may, in whole or in part,
contain or be contained by a previously locked section for the same
process. When this occurs, or if adjacent locked sections would occur,
the sections shall be combined into a single locked section. If the
request would cause the number of locks to exceed a system-imposed
limit, the request shall fail.
.P
F_LOCK and F_TLOCK requests differ only by the action taken if the
section is not available. F_LOCK shall block the calling thread until
the section is available. F_TLOCK shall cause the function to fail if
the section is already locked by another process.
.P
File locks shall be released on first close by the locking process of
any file descriptor for the file.
.P
F_ULOCK requests may release (wholly or in part) one or more locked
sections controlled by the process. Locked sections shall be unlocked
starting at the current file offset through
.IR size
bytes or to the end-of-file if
.IR size
is (\fBoff_t\fR)0. When all of a locked section is not released (that
is, when the beginning or end of the area to be unlocked falls within a
locked section), the remaining portions of that section shall remain
locked by the process. Releasing the center portion of a locked section
shall cause the remaining locked beginning and end portions to become two
separate locked sections. If the request would cause the number of
locks in the system to exceed a system-imposed limit, the request
shall fail.
.P
A potential for deadlock occurs if the threads of a process controlling
a locked section are blocked by accessing a locked section of
another process. If the system detects that deadlock would occur,
\fIlockf\fR()
shall fail with an
.BR [EDEADLK] 
error.
.P
The interaction between
\fIfcntl\fR()
and
\fIlockf\fR()
locks is unspecified.
.P
Blocking on a section shall be interrupted by any signal.
.P
An F_ULOCK request in which
.IR size
is non-zero and the offset of the last byte of the requested section is
the maximum value for an object of type
.BR off_t ,
when the process has an existing lock in which
.IR size
is 0 and which includes the last byte of the requested section, shall be
treated as a request to unlock from the start of the requested section
with a size equal to 0. Otherwise, an F_ULOCK request shall attempt to
unlock only the requested section.
.P
Attempting to lock a section of a file that is associated with a
buffered stream produces unspecified results.
.SH "RETURN VALUE"
Upon successful completion,
\fIlockf\fR()
shall return 0. Otherwise, it shall return \-1, set
.IR errno
to indicate an error, and existing locks shall not be changed.
.SH ERRORS
The
\fIlockf\fR()
function shall fail if:
.TP
.BR EBADF
The
.IR fildes
argument is not a valid open file descriptor; or
.IR function
is F_LOCK or F_TLOCK and
.IR fildes
is not a valid file descriptor open for writing.
.TP
.BR EACCES " or " EAGAIN
.br
The
.IR function
argument is F_TLOCK or F_TEST and the section is already locked by
another process.
.TP
.BR EDEADLK
The
.IR function
argument is F_LOCK and a deadlock is detected.
.TP
.BR EINTR
A signal was caught during execution of the function.
.TP
.BR EINVAL
The
.IR function
argument is not one of F_LOCK, F_TLOCK, F_TEST, or F_ULOCK; or
.IR size
plus the current file offset is less than 0.
.TP
.BR EOVERFLOW
The offset of the first, or if
.IR size
is not 0 then the last, byte in the requested section cannot be
represented correctly in an object of type
.BR off_t .
.P
The
\fIlockf\fR()
function may fail if:
.TP
.BR EAGAIN
The
.IR function
argument is F_LOCK or F_TLOCK and the file is mapped with
\fImmap\fR().
.TP
.BR EDEADLK " or " ENOLCK
.br
The
.IR function
argument is F_LOCK, F_TLOCK, or F_ULOCK, and the request would cause
the number of locks to exceed a system-imposed limit.
.TP
.BR EOPNOTSUPP " or " EINVAL
.br
The implementation does not support the locking of files of the type
indicated by the
.IR fildes
argument.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Locking a Portion of a File"
.P
In the following example, a file named
.BR /home/cnd/mod1
is being modified. Other processes that use locking are prevented from
changing it during this process. Only the first 10\|000 bytes are
locked, and the lock call fails if another process has any part of this
area locked already.
.sp
.RS 4
.nf

#include <fcntl.h>
#include <unistd.h>
.P
int fildes;
int status;
\&...
fildes = open("/home/cnd/mod1", O_RDWR);
status = lockf(fildes, F_TLOCK, (off_t)10000);
.fi
.P
.RE
.SH "APPLICATION USAGE"
Record-locking should not be used in combination with the
\fIfopen\fR(),
\fIfread\fR(),
\fIfwrite\fR(),
and other
.IR stdio
functions. Instead, the more primitive, non-buffered functions (such as
\fIopen\fR())
should be used. Unexpected results may occur in processes that do
buffering in the user address space. The process may later read/write
data which is/was locked. The
.IR stdio
functions are the most common source of unexpected buffering.
.P
The
\fIalarm\fR()
function may be used to provide a timeout facility in applications
requiring it.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIalarm\fR\^(\|)",
.IR "\fIchmod\fR\^(\|)",
.IR "\fIclose\fR\^(\|)",
.IR "\fIcreat\fR\^(\|)",
.IR "\fIfcntl\fR\^(\|)",
.IR "\fIfopen\fR\^(\|)",
.IR "\fImmap\fR\^(\|)",
.IR "\fIopen\fR\^(\|)",
.IR "\fIread\fR\^(\|)",
.IR "\fIwrite\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<unistd.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
